1. Field of Invention
The present invention relates to a phase change memory, and more particularly to a lateral phase change memory with spacer electrodes and method of manufacturing the same, wherein the phase change memory has electrodes with a smaller contact area formed therein, so as to reduce the required programming current for the phase change.
2. Related Art
Many different materials have two or more states, and materials with two states are particularly suitable for being used in digital memory. The state of the chalcogenide materials can be changed in a thermal inductive way as the temperature changes, so as to be in an amorphous state or a crystalline state.
Basically, when the chalcogenide material is in the amorphous state, i.e., an irregularly arranged atomic lattice, it has a higher resistivity; and when the chalcogenide material is in the crystalline state, i.e., a regularly arranged atomic lattice, it has a lower resistivity. The amorphous state, also referred as the reset state, can represent logic 1; whereas the crystalline state, also referred as the set state, represents logic 0. Moreover, the structural state of chalcogenide material can stably exist in an environment with a temperature below the glass transition temperature; therefore, the phase change memory element can be considered as a non-volatile programmable resistor, which can be reversibly and alternately changed between high resistance and low resistance.
The phase change of chalcogenide material is rapid and reversible, and the resistance difference between the amorphous state and crystalline state caused by phase change is significant; therefore, the chalcogenide material is particularly suitable for being used as the material of the phase change memory. In general, the phase transition of chalcogenide material from the amorphous state to the crystalline state can be finished in nanoseconds, and the difference between the corresponding high resistance and low resistance can be higher than six orders of magnitude.
The structure of chalcogenide material changed as the temperature changes in the phase change memory is determined by the current level which passes through the heating electrode and causes the ohmic heating effect, wherein the heating electrode is adjacently connected to the body of the chalcogenide material. The heating electrode is mainly formed by a conductive material with higher resistivity, and the current density can be increased by reducing the contact area between the heating electrode and the chalcogenide material. Therefore, the heating efficiency can be increased and the programming current can be reduced. The operation of phase change memory element can be accomplished by applying two different programming pulses to heat the localized phase change material to an elevated temperature. Depending upon the elevated temperature the material is melted to form either the amorphous state or the crystalline state by modifying the amplitude and duration of the programming pulses, i.e., so-called reset operation and set operation, respectively. The programmed states of phase change memory can be memorized by reading the resistance difference between these two states.
Generally, the memory cell design of the conventional phase change memory array employs 1T1R architecture; that is, the phase change memory element is connected with the steering transistor in series, wherein the phase change memory element is stacked on the transistor and connected with the drain of the transistor. The process of the phase change memory element is compatible with existing CMOS standard processes, and is primarily added to the back-end process. The steering transistor can be used as the cell selector while reading and programming the phase change memory element, and the current required to program the phase change memory element must also flow through it. So, the size of the transistor should be large enough to support the programming current of phase change memory, and its size also dominates the area of the memory cell in the phase change memory unit. Therefore, the density of the phase change memory can be efficiently increased by reducing the programming current of the phase change memory, which also has become the biggest challenge for the development of the phase change memory technology.
Further, the MOSFET transistor is the most common steering device in the phase change memory technology. The area of the phase change memory unit is mainly limited by the area of the MOSFET transistor, such that in order to increase the density of the phase change memory, the programming current should be reduced so as to reduce the size of the MOSFET transistor. The programming current can be reduced by enhancing the heating efficiency of the electrode, and the heating efficiency of the electrode can be enhanced generally by two ways. One way is reducing the contact area between the heating electrode and the phase change material, thus the current density can be increased; the other is using a heating electrode material with higher resistivity to further enhance the ohmic heating efficiency.
For example, the phase change memory technology issued in Symposium on VLSI Technology 2003 in the year of 2003 discloses a phase change memory formed through the edge contact process, wherein the contact area between the phase change memory and the heating electrode can be controlled by the thickness of the deposited heating electrode. Compared with the conventional structure in which the contact area is limited by the ability of the lithography process, this method can obtain a tremendous breakthrough for reducing the contact area.
However, the heating electrode in the disclosed edge contact phase change memory cell is located in the sandwiching layer of the side walls of the trench. This may result in the gap-filling and sidewall-contacting difficulty of the phase change material as well as the problems of uniformity and reliability. Further, the current flowing path of the heating electrode with higher resistivity in the phase change memory is relatively long, and the phase change material occupies an excessively large volume in the element; thus, when the current flows form the lateral heating electrode to the upper electrode, more power will be consumed.
In addition, a lateral phase change memory and method therefor are disclosed in U.S. Pat. No. 6,867,425 in Mar. 15, 2005, wherein an electrode material is formed on a substrate and then patterned, and the patterned electrodes are used as the two electrodes on the phase change material through which the current is flowing. The benefit is that the electrode contact area can be reduced by the lateral contact so as to reduce the programming current, and the path for the current to flow through the phase change material can be reduced by shortening the distance between the two electrodes, thereby reducing the power consumption of the element during operation. Generally, to increase the heating efficiency of the phase change material, a heating electrode material with higher resistivity is required to be used in the phase change memory element, and if the heating electrode is also used as a conductive path in the design, the parasitic resistance will be increased resulting in additional power consumption. Further, when the distance between the two electrodes has been gradually reduced, it will be more and more difficult for the gap filling of phase change material, thereby resulting in poor interface contact between the lateral electrode and the phase change material, and problems of uniformity and reliability of the element.
Accordingly, it is necessary to provide a phase change memory with smaller contact area, lower programming current, and lower power consumption, to overcome the drawbacks of the conventional art.